Coral reefs face desperate struggle

"If ecosystem health declines further, dissolution will overtake growth, and coral skeletons will begin dissolving into the water column, resulting in coral death.

"Some reefs already exhibit net-dissolution, such as in a case study in Florida on an impacted reef.

"We show in this study that ecosystem photosynthesis of marine algae is increasing concurrently with declining calcification, indicating a shift in ecosystem functionality toward algal domination, which can result in lower reef health, resilience and biodiversity."

Mike van Keulen, senior lecturer in plant sciences and marine biology at Murdoch University in Western Australia, said there is no question that the world's tropical reefs are in danger.

"Climate change is the global, overarching issue," he said. "As temperatures rise, the corals are more vulnerable to bleaching. Healthy reefs should be able to largely recover from bleaching events, so long as they're not too frequent."

He cited Ningaloo Reef off the northwest coast of Western Australia as an example.

Located in the 705,015-hectare Ningaloo Coast World Heritage Site 1,200 km north of Perth, the reef was severely bleached in 2011, but has since recovered.

"It becomes more complicated when other local issues are also in play, such as overfishing, pollution and sedimentation," van Keulen said.

"These smaller-scale, more-local factors undermine the resilience of coral reef ecosystems, which means they don't have the resources to bounce back after things like bleaching events.

"This is why actions like Marine Protected Areas are so important－they help protect the ecosystem from these local impacts, giving them more resilience to larger-scale impacts like climate change."

A study by the United States National Academy of Sciences released in May said that should global warming continue at its current rate, the fate of coral reefs worldwide remains grim.

It said the reefs will stop growing in the next decade or so unless a significant reduction in greenhouse gases is achieved.

The study found that in the worst-case scenario presented by researchers, 94 percent of reefs could erode by 2050. Under other scenarios, declines are projected to be so severe that reef production will cease by 2100, the researchers said.

Van Keulen said: "Of course, climate change is accelerating, as we've seen on the Great Barrier Reef. With several years of bleaching in quick succession, there's little chance for the ecosystem to recover.

"I think we are now at the stage where permanent or very long-lasting damage has been done to many of our reefs. We have already lost a lot of the diversity and complexity of coral reef ecosystems around the world.

"We are likely to still have coral reefs into the future, but they won't look the same as they have traditionally－we'll get changes in the types of corals that dominate, from the delicate forms to more-robust forms that are better able to withstand bleaching.

"The sooner we stop the temperature increases associated with climate change, the more chance we'll have of retaining some semblance of our existing reefs."